The veterinary mystery of the California cluster

Earlier this year, The Blood-Horse and the Daily Racing Form both reported that California Thoroughbred racing at Hollywood Park and Santa Anita saw a spike in presumptive sudden cardiac deaths from July 1, 2011, to June 30, 2012. The fatalities occurred both during training and racing. Eleven horses died of cardiac failure during the one-year period, compared with six during the similar period in fiscal year 2011 and four in 2009, according to the California Horse Racing Board's annual reports (see "A well-documented mystery").

"Sudden death in a racehorse is distressing for everyone involved in racing," says Peter Physick-Sheard, BVSc, MSc, FRCVS, associate professor at the University of Guelph (Ontario), Veterinary College, who has been studying the issue in both standardbred and thoroughbred racehorses, looking at electrocardiograms and heart rhythms in horses while they're racing. "It raises animal welfare, economic and safety concerns and represents horrendous public relations for the industry."

Sudden deaths in horseracing are not uncommon, but the recent cluster of inexplicable cardiac deaths in thoroughbreds in California continues to stump researchers and key figures in the industry. (GETTY IMAGES/ANETT SOMOGYVÁRI)

Although sudden death in thoroughbred racehorses is fairly rare, it is of greater significance than in racing standardbreds. A study published in the Equine Veterinary Journal on sudden cardiac death in racing thoroughbreds found that it occurred in 9 percent of fatalities in California—96 cases in exercising horses between February 1990 and August 2008.1 Also during that 18-year period, five cases were reported in Pennsylvania; 23 in Victoria, Australia; 16 in Sydney; four in Hong Kong; and none in Japan.

"Sudden deaths have been an issue in horseracing internationally for many years," says Rick Arthur, DVM, equine medical director at the UC-Davis School of Veterinary Medicine, which was assigned to advise the California Horse Racing Board. "They are typically called heart attacks, but, in reality, cardiac failure, or even suspicious cardiac failure, accounts for less than 50 percent of all fatalities."

Some of the so-called "cardiac sudden death cases" may actually be pulmonary hemorrhage, Arthur says. "It is common for horses to bleed externally due to pulmonary hemorrhage after training, or there may be a major vessel rupture internally," he says.

Though cardiopulmonary-vascular incidents of those types are some of the most common, in California we've had a number of sudden deaths from other issues. About 10 percent of them are the result of a major bone fracture. In some of those incidents, the horse goes down and exsanguinates internally, so it is at first deemed a cardiac death, though that may not be the cause at all."